This invention relates to a collision sensor which is adapted to sense acceleration applied thereto in the event of collision of a vehicle and actuate an occupant protective system such as an inflatable air-bag system upon sensing of the acceleration.
Conventionally, a collision sensor of this kind has been proposed e.g. by Japanese Utility Model Registration Publication (Kokoku) No. 53-39263, which has a rotary plate having a center of gravity eccentric to an axis thereof such that the rotary plate rotates in response to acceleration applied thereto when a vehicle is involved in a crash, for instance, and a switch mechanism operable to actuate an occupant protective system when the rotary plate rotates by a predetermined angle.
According to the proposed device, however, the rotary plate is normally biased in a reference position by a spiral spring so that the force of the spiral spring increases as the rotary plate rotates away from the reference position to thereby increase its urging force acting on the rotary plate. That is, as the rotary plate rotates from the reference position by a predetermined angle, the force of the spiral spring correspondingly increases from the minimum value to the maximum value. Therefore, the proposed device had the disadvantage that there is a limitation in selecting a spiral spring having a suitable spring constant for the degree of acceleration which should occur in the event that the vehicle is involved in a crash, for instance, thereby spoiling the freedom of design or selection of the spiral spring. This problem is still encountered even if a leaf spring is used in the collision sensor in place of the spiral spring.